20.职责链模式.md

职责链模式 Chain of Responsibility

• 对象的职责分派将更具灵活性，可以在运行时动态添加\修改请求的处理职责
• 如果请求传递到职责链的末尾仍得不到处理，应该有一个合理的缺省机制

动机

在软件构建过程中，一个请求可能被多个对象处理，但是每个请求在运行时只能有一个接受者，如果显式指定，将必不可少的带来请求发送者与接收者的紧耦合

模式定义

使多个对象都有机会处理请求，从而避免请求的发送者和接收者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递请求，直到有一个对象处理它为止。

代码

enum class RequestType{
    REQ_HANDLER1,
    REQ_HANDLER2,
    REQ_HANDLER3
};

class Request{
    string description;
    RequestType reqType;
public:
    Request(const string& desc, RequestType type) : description(desc), reqType(type){}
    RequestType getReqType() const {return reqType;}
    const string& getDiscription() const {return description;}
};

class ChainHandler{
    ChainHandler* nextChain;
protected:
    virtual bool canHandleRequest(const Request& req) = 0;
    virtual void processRequest(const Request& req) = 0;
public:
    ChainHandler(){
        nextChain = nullptr;
    }
    void setNextChain(ChainHandler* next){
        nextChain = next;
    }
    void handle(const Request& req){
        if(canHandleRequest(req)){
            processRequest(req);
        }else{
            if(nextChain != nullptr){
                nextChain->handle(req);
            }
        }
    }
};

class Handler1 : public ChainHandler{
protected:
    bool canHandleRequest(const Request& req) override {
        return req.getReqType() == RequestType::REQ_HANDLER1;
    }
    void processRequest(const Request& req) override {
        cout << "Handler1 is handle request: " << req.getDiscription() << endl;
    }
};
class Handler2 : public ChainHandler{
protected:
    bool canHandleRequest(const Request& req) override {
        return req.getReqType() == RequestType::REQ_HANDLER2;
    }
    void processRequest(const Request& req) override {
        cout << "Handler2 is handle request: " << req.getDiscription() << endl;
    }
};
class Handler3 : public ChainHandler{
protected:
    bool canHandleRequest(const Request& req) override {
        return req.getReqType() == RequestType::REQ_HANDLER3;
    }
    void processRequest(const Request& req) override {
        cout << "Handler3 is handle request: " << req.getDiscription() << endl;
    }
};

int main()
{
    Handler1 h1;
    Handler2 h2;
    Handler3 h3;
    h1.setNextChain(&h2);
    h2.setNextChain(&h3);

    Request req("process task ...", RequestType::REQ_HANDLER3);
    h1.handle(req);
    return 0;
}

题目

• 职责链模式-请假审批